Conductive device



14, 1941. PEARSON 2,258,958

CONDUCTIVE DEVICE Filed July 13, 1939 FIG.

- INVENTOR G. L. PEARSON BY ATTORNEY Patented Oct. 14, 1941 2,258,958 v CONDUCTIVE navroa Gerald L. Pearson, Towaco, N. 3.,

Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 13, 1939, Serial No. 284,164

9 Claims.

This invention relates to electrically conductive devices and more particularly to such devices having a high temperature coefllcient of resistance.

In the regulation and control of electrical current for various purposes, it is often advantageous to employ circuit elements whose conductance varies as a function of the applied voltage or current. Resistor elements, the resistance of which is highly dependent upon temperature, are illustrative of such devices. The so-called semiconductors, which have resistance values intermediate those normally associated with conductors and insulators, in most cases have a high negative temperature coeilicient of resistance at ordinary temperatures. Circuit elements made from such materials have therefore been used for voltage and current regulation. Materials ordinarily classed as conductors in most cases have a positive temperature coefllcient of resistance. The value of this coefllcient is low as compared with that of the negative coefllcient materials. For example, iron does not double in resistance for less than 70 C. rise in temperature at any point below 500 C. Such materials, therefore, are not available for circuit elements which must have a wide range of resistance variation.

One object, therefore, of this invention is to produce a conductive device having a high temperature coefllcient of resistance.

A further object of thi invention is the production of a resistor unit having a high positive resistance-temperature coefllcient.

One feature of this invention resides in a conductive device comprising a plurality of conducting particles supported by a body of nonconductive material, whereby adjacent particles are in mutual contact, the particles and body having widely divergent thermal coefficients of expansion.

In accordance with another feature of this invention, the conductive device comprises conducting particles suspended in an insulating tube, the particles and tube having widely separated thermal coeflicients of expansion.

A further feature of this invention resides in a conductive device comprising conducting particles having a low thermal expansion coemcient and supported by a matrix having a high thermal coeflicient of expansion.

Other and further objects and features of this invention will be more clearly and fully understood from the following detailed description with reference to the appended drawing in which:

Fig. 1 is a sectional view of one type of conductive unit illustrative of this invention;

la. 2 is a sectional view of another illustrative embodiment of this invention and Figs. 3 and 4 are respectively a section and a plan view of a further form of conductive unit illustrating this invention.

Certain parts shown in the drawing have been purposely exaggerated to better illustrate the invention. For example, the spherical conducting particles which may be very small, often being in the order of 3.5 mils diameter, have been shown out of proportion in the interests of clarity.

The conductive units may, in accordance with this invention, comprise conducting particles suspended in an insulating body or matrix which has a coei'iicient of thermal expansion widely different from that of the particles. The particles are so suspended by the body or matrix that each-is in contact with the particles thereadJacent. That is, the mutual contact is such that variations in pressure change the contact resistance between particles in a manner similar to that found in a carbon granule type microphone. A variable resistance, interparticle contact of this type is sometimes called a microphonic or modulating contact.

The relation between the coefficients of thermal expansion of the particles and supporting body determines the sign of the temperature coemcient of resistance of the device. With particles of low thermal coefllcient and a supporting body or matrix of high thermal coefllcient, the device has a positive resistance-temperature coeiiicient. Conversely, with high expansion particles and a low expansion matrix, the resistance-temperature coefllcient is negative. The value of the resistance-temperature coeilicient or half temperature is a function of the diiference between the thermal expansion coefiicients of the conductive and non-conductive elements. The term half temperature may be defined as the'temperature range over which the resistance doubles or is reduced by half its original value.

The temperature range over which the half temperature is measured should be specified. It is convenient in many cases to employ the range 0 to 25 C. for half temperature measurements and such range will be understood in this application unless otherwise specified.

Referring now to the drawing and particularly to Fig. 1, it designates a plurality of conducting granules enclosed in a tube ll of insulating material, Conductors l2, which serve as terminals or electrodes for the unit, are sealed into the ends of tube H to complete the enclosure. If the device is to have a negative resistance-temperature co'eiiioient, the particles Ill should have a high and tube l,l a low thermal expansion coemcient. For example, the particles may be of carbon and the tube of quartz. To obtain a positive temperature coeflicient of resistance, low expansion conductive particles, such as carbon coated quartz spheres and a high expansion tube, made for example of soft glass, may be used. Conductors I2 should have a thermal expansion coefiicient as close to that of the tube II as possible. For soft glass tubes, platinum conductors are suitable. In view of the difliculty oi sealing metals to quartz, other methods may be employed for securing conductors l2 thereto. The sealing-in is a convenient method of securing the conductors to the tube, so that there is no appreciable relative movement therebetween. The conductors 12 may be secured to the tube I I by cementing and like means in situations where such methods are more satisfactory than sealing-in.

Where conductors [2 are sealed in the tube II, the temperatureof the device at the time of sealing-in the second conductor, determines the initial pressure on the enclosed particles. Since the temperature at which a high resistance-temperature coefficient is evident, is a function of the initial pressure, said pressure should be adjusted in accordance with the characteristics desired in the completed unit.

. Other conductive units of the type illustrated in Figs. 2, 3 and 4 may be made by maintaining a plurality of conducting particles in microphonic contact by a matrix of plastic material. The so-called organic plastics, most of which have a high thermal expansion coeflicient, provide suitable matrix materials for high positive resistance-temperature coeiiicient conductive devices. However, these materials must have a sufiiciently high resistance to cold flow to return to their original dimensions at the end of successive heating and cooling cycles. This is necessary in order to obtain stable conductive units. Someof such materials are metal stearates, for example, aluminum steal-ate, phenol resins, methyl methacrylate, cellulose acetate, cellulose nitrate and the polystyrenes.

One form of positive resistance-temperature coefliclent unit of the plastic matrix type is il-- lustrated in Fig. 2. Here the conducting particles 20 are molded into a matrix 2| of insulating material. Circuit connections may be made to the unit in various ways. bodiment, conductive leads 22 are attached to the'unit by bolts 23 which'serve as electrodes.

The leads may also be molded in the material or melted in by the application of heat. Among the materials giving satisfactory results in vdevices of this form, are iron filings in an aluminum stearate matrix. Various types of iron, in finely divided form, may he used. For example, a 36 per cent nickel steel known as Invar and having a very low thermal expansion coefllcient has been used successfully. The iron filings may be mixed with powdered metal stearate and the material molded intoa block or disc. Other par ticle and matrix materials having suitable characteristics may also be employed in this manner.

Units of the type shown inFigs. 3 and 4 may be made by securing conducting. particles to the In the illustrated em- 7 such as carbon coated quartz spheres, 30 to the surface of a body of organic plastic material 3|. One method of doing this is to spread a layer of the spheres over one face of the plastic body and embed them therein by the application of 7 heat and pressure. -Metal electrodes 32 may be applied to the spaced portions of the layer of spheres 30. The electrodes may be applied by any suitable method, for example, by means of a metal spray such as a Schoop spray. Conductors 33 may be attached to the electrodes 32 by solder or like means. Another method of applying the layer 30 is to mix a quantity of the coated spheres with some of the plastic material dissolved in a solvent and to apply the resulting mixture to a surface of the plastic body. The conductive portion of this type of unit comprises essentially a single layerof. conducting spheres in which adjacent spheres are substantially mutually tangent.

Half temperatures of the order of 3 to 5 C. may be obtained in positive resistance-temperature coeificient devices made in accordance with this invention. Such devices may be used for regulationand like applications in a manner similar to that employed with negative resistance-temperature coefficient means of various types. In general, the positive coefiicient units are connected in series to accomplish the same results as negative coefiicient units connected in parallel and vice versa.

Various particle and matrix combinations not specifically indicated in the foregoing description may be employed in carrying out this invention. For example, carbon particles if used with a high expansion matrix such as \one of the before-noted plastics would give a positive resistance-temperature coeflicient device; iron filings may be attached to the surface of a plastic body instead oi being molded therein or carbon coated quartz spheres may be incorporated throughout 7 insulating material, whereby voids among said particles ar filled with said material,- said particles being held in mutual, microphonic contact by said material, said material having a. thermal expansion ooeflicient widely different from that of said particles. a

2-. A conductive device having a high positive resistance-temperatin'e coeflicient comprising a plurality of conducting particles. and a plurality of electrodes secured to a body of insulating materiaihaving a high themal'expansion coeflicient as compared with that of said particles, each particle being embedded in said material, said particles being held in mutual, microphonic contact between the electrodes by said material.

3. A conductive device having a high positive resistance-temperature coeflicient comprising a plurality of particles of iron, a matrix' of aluminum stearate, and a-piurality of electrodes, the matrix maintaining the electrodes in spaced relation and supporting the particles so that contiguous particles are in mutually contacting relation and the particles adiacentsald electrodes ar in contact therewith.

4. A conductive device having a high posimutually contacting relation, and electrodes attached to spaced portions of said layer.

7. A high resistance-temperature coemcient conductive device-comprising conductive particles and insulating material having widely divergent thermal expansion coeflicients, said material embedding said particles in mutually contacting relation and filling voids ereamong,

' whereby theinterparticle contact pressure varies resistance-temperature eoeiiicient comprising a plurality of carbon coated quartz spheres, a matrix of cellulose acetate plastic, and a plurality or electrodes, the matrix maintaining the electrodes in spaced relation and supporting the spheres so that contiguous spheres are in mutually contacting relation and the spheres adjacent said electrodes are in contact therewith. 6. A conductive device having a high positive resistance-temperature coeflicient comprising a plurality oi. carbon coated quartz spheres and a matrix of cellulos acetate p1astic,'the'matri having a layer of spheres attached to one surface thereot so that contiguous spheres are in with temperature to vary the resistance or said device.

8. A conductive device comprising a plurality of conducting particles embedded in organic plastic, insulating material,v said material occupying voids among the particles, said particles being 'held' in mutual microphonic contact by said material. said material having a very high thermal expansion coemcient as compared to that of the particles.

9. A high resistance-temperature coemcient conductive device comprising conducting particles, embedded in an insulating material so that adjacent particles are in mutual contact and voids among the particles are filled with said material, said insulating material having a thermal expansion coemcient greatly diiierent from that o! the particles, whereby the interparticle contact pressure and hence resistance varies widely with changes in temperature.

GERALD L. PEARSON. 

